The present invention relates to an acoustic touch position sensor and more particularly to a touch panel of the type wherein an acoustic wave is generated within a substrate, the acoustic wave propagating in the substrate having a range of characteristic time delays from a transmitted signal, representing the differing path lengths associated with each axial displacement along an axis of the substrate. A touch on the substrate results in a perturbation of the wave, which is sensed to determine the axial displacement of the touch on the substrate. Touch panels of this type are used as computer input devices in connection with computer image displays.
Conventional touch panels are utilized as input-output devices, applicable in various fields, in combination with a display device or unit such as a cathode ray tube (CRT), a liquid crystal display (LCD) or a plasma display panel (PDP). Resistive, capacitive, and acoustic touch panels are presently the dominant types of touch panels in the market-place. Acoustic touch panels provide a more robust touch surface and greater image clarity than resistive and capacitive touch panels.
Resistive and capacitive touch panels include a resistance layer formed on a substrate. Due to its strength, optical clarity, and low cost, soda-lime glass is generally the preferred substrate material. The resistance layer is essential for the detection of touch position information. In addition, a conventional resistive touch panel includes an overlaying plastic cover sheet. For many applications, such added components to the glass substrate may be susceptible to accidental or malicious damage. Furthermore, these added components degrade the visibility of data and images in a display device as a result of decreased light transmission and increased reflection of ambient light.
In contrast, conventional acoustic touch panels can be advantageously employed in order to insure a robust touch surface and an enhanced display image quality. Because ultrasonic acoustic waves are used to detect coordinate data on input positions, a resistance layer need not be formed on the glass soda-lime substrate and no plastic cover sheet is required. Soda-lime glass is quite transparent and supports propagation of acoustic waves at ultrasonic frequencies. Soda-lime glass is the substrate material of conventional acoustic touch panels. For the end user, such an acoustic touch panel is optically and mechanically little more than a piece of windowpane glass.
Typically, 4% of incident light is reflected off each glass surface resulting in a maximum light transmission of about 92%. Reflection of ambient light reduces image contrast. These reflections are caused by the index-of-refraction mismatch between air and the glass substrate. Decreased light transmission reduces image brightness. These can be important effects when a touch panel is placed in front of a display device having a relatively low luminance (brightness) such as a liquid crystal display. Known methods for reducing reflections and increasing transmission are optical bonding or anti-reflective coatings. These methods address the index-of-refraction mismatch between air and glass. These methods do not improve the inherent transparency of the substrate material itself.
Soda-lime glass is not completely transparent. This is mainly due to color centers caused by iron ion impurities.
These iron impurities decrease light transmission and distort the colors of displayed images. These are minor effects relative to, for example, the optical differences between acoustic and resistive touch panels. Nevertheless, improved transmission relative to common soda-lime glass would provide a useful enhancement of the optical advantages of acoustic touch panels.
Display technology is evolving rapidly. This evolution includes introduction and market acceptance of large sized display products. This in turn creates demand for larger touch panels. However, all touch panel technologies encounter problems when scaled to larger sizes. For resistive and capacitive touch panels, it becomes more difficult to maintain sufficient uniformity in resistance layers as panel sizes increase. For acoustic touch panels, the challenge for larger sizes is to assure sufficient signal amplitudes.
For acoustic touch panels, acoustic signals decrease as panel dimensions increase. This signal loss occurs because of the attenuation or damping of the ultrasonic waves as they propagate through the substrate. Thus, large-sized acoustic touch panels may fail to provide sufficient signal-to-noise ratio to reliably determine input positions. Hence there is a need for means to enhance the signal-to-noise ratio for acoustic touch panels. This is all the more true because there are other market pressures for product enhancements that reduce signal amplitudes: lower-cost controller electronics; reduced area reflective arrays; signal-absorbing seals; etc.
Due to the relatively long acoustic path lengths of commercially successful acoustic touch panel designs, acoustic attenuation properties of the glass substrate are particularly important. To understand the need for long acoustic path lengths, consider this first and simplest concept for acoustic touch panels.
Conceptually, the simplest acoustic touch position sensor is of the type described in U.S. Pat. No. 3,673,327. Such touch panels includes a plate having an array of transmitters positioned along one edge of a substrate for generating parallel beams of acoustic waves. A corresponding array of receivers is positioned along the opposite edge of the substrate. Touching the panel at a point causes attenuation in one of the beams of acoustic waves. Identification of the corresponding transmitter/receiver pair determines a coordinate of the touch. The acoustic touch panel disclosed in U.S. Pat. No. 3,673,327 uses a type of acoustic wave known as a xe2x80x9cRayleighxe2x80x9d wave. These Rayleigh waves need only propagate from one edge of the touch panel to the other. However, note that this type of acoustic touch panel requires many transducers, and hence associated cable conductors and electronics channels. This type of acoustic sensor has never been commercialized due to the expense of providing a large numbers of transducers.
Now consider acoustic touch panels that have been commercially successful. Representative of a set of pioneering patents in this field is Adler, U.S. Pat. No. Re. 33,151. An acoustic transducer generates a burst of waves that are coupled into a sheet-like substrate. These acoustic waves are deflected 90xc2x0 into an active region of the system by an array of wave redirecting gratings. The redirecting gratings are oriented at 45xc2x0 to the axis of propagation of waves from the transducer. These gratings are analogous to partially silvered mirrors in optics. Acoustic waves after traversing the active region are, in turn, redirected by another array of gratings towards an output transducer. A coordinate of the location of a touch is determined by analyzing a selective attenuation of the received signal in the time domain, each characteristic delay corresponding to a coordinate value of the touch on the surface. Use of the arrays of gratings greatly reduces the required number of transducers, thus making possible acoustic touch panels at commercially competitive prices. On the negative side, this clever use of grating arrays considerably increases the maximum distance acoustic waves must propagate through the substrate.
Signal amplitudes in acoustic touch panels are further decreased by inefficiencies in the scattering process at the grating arrays. Such inefficiencies can be minimized through proper array design. Efficient coherent scattering from the arrays is achieved by orienting the grating elements at a 45xc2x0 angle and spacing them at integral multiples of the acoustic wavelength. Most efficient use of acoustic energy is provided when the acoustic power xe2x80x9cilluminatingxe2x80x9d the active area is equalized. Known techniques compensate for the tendency for signal amplitudes to exponentially decay as a function of delay time. As described in lines 37 to 41 of column 11 of U.S. Pat. No. 4,746,914, signal equalization can be achieved with a constant wavelength spacing of the grates, i.e., reflecting elements, by providing reflecting elements which vary in height. An alternative method is to selectively drop grating elements to produce an approximately constant acoustic power density over the active area. In this case, the spacing between the grates decreases with increasing distance away from the transducer along the axis of the array. Applying these known methods avoids unnecessary inefficiencies in redirecting the acoustic waves. Nevertheless, the use of grating arrays to twice redirect the acoustic waves inevitably leads to signal losses. This increases the importance of minimum signal amplitude requirements in acoustic touch panel design.
The electronics for commercially available acoustic touch panel products are based on the basic concepts presented in Brenner et al., U.S. Pat. No. 4,644,100. This patent concerns a refinement of the system according to the U.S. Pat. No. Re. 33,151, wherein perturbations of a received signal are determined by comparing the received signal to a stored reference signal profile. By analyzing both the time delays and of the signal perturbations, the touch sensitive system employing acoustic waves is responsive to both the location and magnitude of a touch. Proper operation of the touch system requires a sufficiently large signal-to-noise ratio to avoid ambiguities between signal perturbations due to an acoustic-wave absorbing touch and signal variations due to electronic noise. Electronic noise may be due to fundamental noise from circuit components or due to electromagnetic interference. In recent years, the marketplace increasingly expects a fast touch response from light touches, which requires lower touch perturbation thresholds, and hence increases demand for a higher signal-to-noise ratio.
Further description of such Adler-type acoustic touch panels may be found in the above cited patents as well as in U.S. Pat. Nos. 4,642,423; 4,644,100; 4,645,870; 4,700,176; 4,746,914 and 4,791,416. For each coordinate axis detected, acoustic waves are generated in, e.g., a glass substrate by a transducer containing a piezoelectric element. Thus, a transmitted wave packet is dispersed along the axis of the transmitting reflective array, traverses the substrate and is recombined into an axially propagating wave by another reflective grating, and is directed to a receiving transducer in a direction anti-parallel to the initial transmitted wave. The wave packet is dispersed in time according to the path taken across the substrate. The received waveform is converted into an electrical signal for processing. The time delay of a perturbation of the electrical signal corresponds to a distance traveled by the perturbed component of the wave. Thus, according to this system, only two transducers per axis are required. Typically both X and Y coordinates are measured; this can be done with a total of only four transducers.
Variations of the above acoustic touch panel systems are possible with further reductions in the numbers of transducers. The acoustic wave may be reflected by 180xc2x0 near or at the edge of the substrate parallel to the axis of the transmission reflective grating and reflected back through the substrate to the reflective array and retrace its path back to the transducer. The transducer, in this case, is configured to act as both transmitter and receiver at appropriate time periods. A second transducer, reflective array and reflective edge are provided for an axis at right angles to allow determination of both coordinates of touch. Still another system provides for a single transducer that produces a wave for detecting a touch on two axes and also receives the wave from both axes. Reducing the number of transducers increases the corresponding acoustic path lengths for a given touch panel size. This increases the signal loss due to acoustic damping within the substrate material.
The touch activating an acoustic touch panel may be due to a finger, gloved or ungloved, or a stylus pressing against the surface. Optionally, the finger or stylus may act indirectly through a cover sheet placed over the glass substrate surface.
There are several modes that ultrasonic waves can take in glass substrates. The mode referred to as a xe2x80x9cRayleighxe2x80x9d wave is of particular interest for acoustic touch panels. Rayleigh waves are essentially confined to a single surface of a sheet of uniform, non-piezoelectric medium of a sufficient finite thickness. Mathematically, Lord Rayleigh calculated the wave function for this mode for a semi-infinite medium. Such waves guided near a surface of a medium of finite thickness are more precisely termed xe2x80x9cquasi-Rayleighxe2x80x9d waves, although such waves are generally referred to as xe2x80x9cRayleigh wavesxe2x80x9d and are so denominated herein. Practical experience with touch panel design and manufacture has shown that about four Rayleigh wavelengths or more is a sufficient substrate thickness to successfully propagate Rayleigh waves.
Other acoustic modes have been investigated for use in acoustic touch panels. U.S. Pat. Nos. 5,260,521; 5,234,148; 5,177,327; 5,162,618 and 5,072,427 disclose the use of horizontally polarized shear waves and Lamb waves in Adler-type acoustic touch panels. U.S. Pat. No. 5,591,945 discloses further options regarding the choice of acoustic modes in acoustic touch panels. Nevertheless, Rayleigh waves have been, and are expected to remain, the most commonly used acoustic mode in acoustic touch panels. This is due to the relatively high sensitivity of Rayleigh waves to touches and due to their ability to be propagated by a simple surface of a homogeneous medium.
For commercial acoustic touch panels, the frequency of the ultrasonic acoustic waves is around 5 MHz. For acoustic touch panels employing Rayleigh waves, the thicknesses of the soda-lime glass substrates for commercial products to date are in the range from 2 mm to 12 mm. Acoustic touch panel products employing lowest order horizontally polarized shear waves are currently made of 1 mm thick soda-lime glass.
Acoustic touch panels, of the type that has proved to be commercially viable, make clever use of reflective arrays to reduce the number of transducers and electronic channels, and to provide a reliable and accurate time-based analog measurement of touch position. This has proved essential to the commercialization of acoustic touch panels. However, the resulting relatively long acoustic path lengths, along with the losses from two acoustic scatters, leads to small received signal amplitudes. With such small signal amplitudes, it is difficult to assure a sufficient signal-to-noise ratio for reliable signal processing in a touch sensor of the type which transmits ultrasonic acoustic waves in a glass substrate.
Many terms have been used to describe acoustic touch panels: xe2x80x9cacoustic sensorsxe2x80x9d, xe2x80x9cacoustic touch screensxe2x80x9d, xe2x80x9cultrasonic touch panelsxe2x80x9d, etc. Unless stated otherwise, all these terms are considered here to be synonyms for a transparent touch sensor which senses touches with ultrasonic acoustic waves and which use reflective arrays of gratings to enable a reduced number of transducers.
There is a need for means to increase signal amplitudes in acoustic touch panels.
It is, therefore, an object of the present invention to provide an acoustic touch panel whose glass substrate has a low rate of acoustic attenuation or damping and which insures an acceptable intensity of transmitted signals.
It is a further object of the present invention to provide an acoustic touch panel which is more reliable and robust with respect to electromagnetic interference than such touch position sensors known heretofore.
It is a further object of the present invention to provide an acoustic touch panel that can operate reliably with a reduced cost controller with transmit-burst amplitudes of about 10 Volts peak-to-peak or less.
It is a further object of the present invention to provide an acoustic touch panel that includes mechanically compact transducers of reduced signal-conversion efficiency.
It is a further object of the present invention to provide an acoustic touch panel that permits the use of seals that cause significant acoustic signal absorption.
It is a further object of the present invention to provide an acoustic touch panel of increased dimensions.
It is a further object of the present invention to provide a touch panel that presents a reliable and robust touch surface to the user even when roughly handled.
It is a still further object of the invention to provide a temperable, low acoustic loss substrate for a touch panel that may be either thermally tempered or chemically hardened, thereby making possible large tempered touch panels.
It is an object of the present invention to increase the signal-to-noise ratio in an acoustic touch panel which employs Rayleigh waves.
Another object of the present invention is to provide a touch panel that insures a high light transmission and clear display of data by the display device.
Extensive research has led to the achievement of the above objects and to the discovery that the use of specific glass substrates or bases as a propagation medium for ultrasonic acoustic waves can suppress attenuation (damping) of the ultrasonic acoustic waves to a great extent, and can also transmit signals while keeping their intensity high until they are received and detected. The present invention is based on the above findings.
These objects, as well as other objects which will become apparent from the discussion that follows, are achieved, in accordance with the present invention by providing a touch panel having a glass substrate as a propagation medium for ultrasonic acoustic waves and which is used for detecting the coordinate data on a position touched. In this glass substrate, comprising SiO2 as the main component, the total content of Na2O, CaO and MgO is 20% by weight or less, and the total content of Al2O3, ZrO2, TiO2, B2O3, Y2O3, SnO2, PbO2, In2O3 and K2O is generally 5% by weight or more.
While this invention was the result of unanticipated experimental results, the following conceptual framework serves to clarify the nature of the invention.
Glass is basically silicon dioxide, SiO2, in which sufficient amounts of other compounds have been added to disrupt the formation of a regular lattice of Sixe2x80x94Oxe2x80x94Si covalent bonds that otherwise would form crystalline quartz. For example, addition of Na2O, results in the replacement of a Sixe2x80x94Oxe2x80x94Si link of covalent bonds between two silicon atoms with a break in the covalent link, Sixe2x80x94Oxe2x88x92/Oxe2x88x92Si, plus two Na+ ions. Similarly, the addition of CaO or MgO results in a break in the covalent link, Sixe2x80x94Oxe2x80x94/Oxe2x80x94Si, plus a Ca2+ or a Mg2+ ion. In this manner, addition of a sufficient amount of xe2x80x9csodaxe2x80x9d and xe2x80x9climexe2x80x9d results in an amorphous glass rather than crystalline quartz.
It is known that the transition from a crystalline to an amorphous material results in increased damping. For example, consider the following translation of a passage from an acoustics textbook of Royer and Dieulesaint (Ondes elastiques dans les solides, Tome 1, page XV, publisher Masson):
Solids used in applications requiring waves of relatively high frequency ( greater than 100 MHz, for example for signal processing) are crystals because mechanical vibrations are attenuated less as the materials in which they propagate are more ordered.
This implies that the use of glass, an amorphous material, rather than a crystalline material like quartz, inevitably results in increased acoustic losses.
The inventors unexpectedly discovered that additions of different compounds to silicon dioxide, all sufficient to induce a transition to an amorphous glassy-state,-vary widely in their effect on acoustic attenuation. Certain glass compositions lead to significantly less acoustic absorption than is present in soda-lime glass. Furthermore, a pattern has been observed.
Acoustic attenuation is relatively larger if the additions replace the Sixe2x80x94Oxe2x80x94Si covalent links with weak ionicbonding links and the acoustic attenuation is relatively smaller if the additions replace Sixe2x80x94Oxe2x80x94Si covalent bonds with alternate covalent bonds, strong ionic bonds, or sterically constrained ionic bonds. The addition of B2O3 leads to Bxe2x80x94Oxe2x80x94Si bonds. It does not result in breaks in the material""s covalent-bond network such as Sixe2x80x94Oxe2x88x92/Oxe2x88x92Si. This is an example of establishing alternate covalent links.
Additions that lead to positive ions of high charge states of three or more, e.g. Al3+ and Zr4+, lead to strong ionic bonds. The oxygen ions at the end of covalent chains, Sixe2x80x94Oxe2x88x92, will form strong ionic bonds with ions of high charge states. Such ionic bonds with high-charge-state ions are strong because the electrostatic binding forces are proportional to the charges of the participating ions. Strong ionic bonds are formed where the Sixe2x80x94Oxe2x80x94Si covalent bonds have been broken.
For additions of the form X2O3 or XO2, it may not be clear whether the element X forms alternate covalent links, Xxe2x80x94Oxe2x80x94Si, or whether the element X forms high charge state ions, X3+ or X4+. In either case, the result is same. The network of molecular bonds is strengthened relative to additions of the form X2O and XO. While this does not make the network any more ordered, it is empirically observed to reduce acoustic attenuation.
While K2O and BaO are of the same form X2O and XO form as Na2O and CaO and MgO, the corresponding ionic radii are very different. The ionic radius of K+ is 1.33 Angstroms and the ionic radius of Ba2+ is 1.35. In contrast, the ionic radii of Na+, Ca2+, and Mg2+ are 0.95, 0.99, and 0.65 respectively. All these ions will be attracted to the negative charges of the negative oxygen atoms terminating the covalent network. However, the large size of, e.g. K+ and Ba2+ ions, relative to, e,g., Na+, Ca2+, and Mg2+ ions, lead to steric effects due to space filling in the region of broken covalent links, Sixe2x80x94Oxe2x80x94/Oxe2x80x94Si. The inventors interpret their observations and discoveries, in part, as due to such steric effects resulting in a suppression of acoustic damping when ionic radii exceed about 1.1 Angstroms.
Steric effects are most pronounced for singly-charged large-radius ions from additions of the form X2O. This is because there are two X+ ions per broken covalent link Sixe2x80x94Oxe2x88x92/Oxe2x88x92Si. K+ is the most important example for X+. Steric effects of K+ ions in glass are known and are the basis for chemically hardening of glass.
The doubly charged ions of larger radii, e.g., Ba2+ and Sr2+, will have stronger steric effects than the smaller doubly charged ions Mg2+ and Ca2+, but will have weaker steric effects than pairs of large singly charged ions like K+, Ba2+ and Sr2+ are more neutral in their acoustic effects.
The above conceptual framework provides a context for the invention specified below.
The touch panel of the present invention is provided with a glass substrate as a propagation medium for the ultrasonic acoustic waves, which is used for detecting the coordinate data on a position touched, whose total content of Na2O, CaO and MgO in the glass substrate is 20% by weight or less and whose total content of Al2O3, ZrO2, TiO2, B2O3, Y2O3, SnO2, PbO2, In2O3 and K2O is generally 5% by weight or more.
The use of such a glass substrate as a propagation medium for the ultrasonic acoustic waves suppresses the attenuation or damping of ultrasonic acoustic waves and insures that a high or an acceptable signal intensity is received.
The touch panel of the present invention also is provided with a glass substrate as a propagation medium for the ultrasonic acoustic waves, which is used for detecting the coordinate data on a position touched and in which the glass substrate has a higher light transmission than a soda-lime glass in the visible ray region.
These objects are further achieved, in accordance with the present invention, by providing an acoustic touch position sensor of the type described above with a substrate made of a transparent material, such as a temperable or tempered glass, preferably a barium-containing glass, which exhibits substantially less acoustic absorption than conventional soda-lime glass.
By xe2x80x9ctemperable glassxe2x80x9d is meant a glass that is capable of being either thermally tempered or substantially chemically hardened.
Thermal tempering occurs when the glass is heated until it is glowing red hot and then rapidly cooled, thereby placing the glass at both surfaces under very high compression because they were cooled so quickly. For fully tempered glass, this may be about 15,000 psi. It is also possible to partially thermally temper the glass to, e.g., about 10,000 psi. The internal portion of the glass cools more slowly, and is under tension, being stretched parallel to the surfaces by both surfaces. Glass can only be heat tempered if it has a sufficiently large thermal expansion coefficient, i.e., has a thermal expansion coefficient greater than about 6xc3x9710xe2x88x926/ K before tempering.
Chemical hardening of glass takes place by the replacement of some of the lower alkali metal ions present at the surface of the glass with ions of higher alkali metals, e.g., the replacement of lithium and/or sodium ions with potassium ions. The chemical hardening process is generally disclosed in U.S. Pat. No. 3,954,487, which is incorporated herein by reference. Here we are interested in glasses that can be xe2x80x9csubstantiallyxe2x80x9d chemically hardened, i.e., to an increase in strength of at least about 50%, preferably to an increase in strength of at least about 100%
It has been discovered, quite unexpectedly, that the use of a temperable barium-containing glass as a substrate for acoustic touch panels that employ Rayleigh waves adds between 10 and 30 dB to the signal-to-noise ratio as compared to equivalent acoustic touch panels using soda-lime glass as the substrate.
On a tonnage basis, the vast bulk of glass produced in the world is soda-lime glass. For example, xe2x80x9cwindow panexe2x80x9d glass is soda-lime glass. Car windows and mirrors are made with soda-lime glass. Being the lowest cost glass material, soda-lime glass is the natural choice for a transparent substrate material. Consequently, all Adler-type acoustic touch panels known to recent dates, excepting this invention, have been based on a glass substrate formed of sodalime glass.
Borosilicate glass was originally developed by Dow Corning and marketed by Corning under the brand name xe2x80x9cPyrex.xe2x80x9d This glass, although somewhat more expensive than soda-lime glass, has found a mass market mainly due to its small coefficient of thermal expansion which enables it to endure large temperature gradients without cracking. Schott Glass also presently markets a borosilicate glass under the brand names xe2x80x9cTempaxxe2x80x9d and xe2x80x9cBoroFloatxe2x80x9d.
In a simple experiment, it has been demonstrated that borosilicate glass is approximately one half as absorptive to Rayleigh waves as soda-lime glass. FIG. 3 illustrates the measurement method used to determine Rayleigh-wave attenuation in glass. A transmit and receive transducer pair, 2 and 4, respectively, was placed on the glass and the distance between them was varied between two inches, four inches and six inches. Measurements were taken with two samples of soda-lime glass and two samples of borosilicate glass at each of the distances. In this case, the borosilicate glass was a sheet of Tempax glass manufactured by Schott. The results are illustrated graphically in FIG. 4.
As may be seen in FIG. 4, the attenuation in the sodalime glass was approximately twice the attenuation measured for the borosilicate glass. The soda-lime glass exhibited an attenuation of 1.44 dB per inch; the borosilicate glass attenuated the same signal by 0.74 dB per inch. Relative to soda-lime glass, these data imply that borosilicate glass has 0.70 dB of additional signal per inch of acoustic path. For a maximum acoustic path length of twenty to forty inches, this implies 14 to 28 dB of additional signal.
Follow-up measurements were made with Schott""s xe2x80x9cBorofloatxe2x80x9d borosilicate glass and soda-lime glass from a variety of sources. The results confirm the advantage of borosilicate over soda-lime glass.
In experiments, all barium-containing glasses tested share the low acoustic loss characteristics of borosilicate glass. An example of a barium-containing glass is the structural element of the faceplate used in the manufacture of the Zenith 1490 FTM (flat tension mask) monitor. Samples measured were observed to have an acoustic attenuation of approximately 0.6 dB/inch. Similar low acoustic attenuation was observed on the faceplates of a variety of cathode-ray tubes of a variety of monitor products: MiniMicro MM1453M; Mitsubishi AUM-1371; Quimax DM-14+; NEC A4040; and Goldstar 1420-Plus. Another example of a barium-containing glass is Schott B-270 glass, which is reported to have the approximate composition (weight % on oxide basis) SiO2: 69.5, Na2O: 8.1, K2O: 8.3, CaO: 7.1, BaO: 2.1, ZnO: 4.2, TiO2: 0.5, Sb2O3: 0.5.
Use of a low-loss glass in an acoustic touch panel according to the invention thus provides an extra measure of signal xe2x80x9cbudgetxe2x80x9d due to the increased signal-to-noise ratio. This increased budget makes it possible to achieve many objectives that, at least superficially, appear to be unrelated to the choice of substrate material. These are enumerated below:
(1) The increased signal-to-noise ratio makes it possible to reduce the cost of the electronic controller associated with the touch panel. In particular, the burst circuit of the controller, which sends a tone burst to the touch panel transmitting transducers, may be simplified by reducing the burst amplitude to, e.g., transistor-transistor logic (TTL) voltage levels, making it possible to use lowercost circuits at the output stage. Reducing the burst amplitude also has the advantage of reducing EMI emissions from the controller.
(2) Acoustic touch panels of the type disclosed in the U.S. Pat. No. Re. 33,151 use reflective arrays to minimize the number of transducers and electronic channels and to provide a reliable and accurate time-based analog measurement of touch position. However, the resulting relatively long acoustic wave path lengths, along with the losses from two acoustic wave scatters, leads to small received signal amplitudes and limits the overall size of the touch panel. An increase in the signal-to-noise ratio, resulting from the use of borosilicate glass or barium-containing glass, makes it possible to increase the overall size of this type of touch panel. For example, rectangular touch panels may have a diagonal dimension of at least twenty-one inches.
(3) It is often necessary to allow contact between the sensitive portion of the touch panel and the adjacent objects. For example, a CRT housing or bezel may make contact with an acoustic touch panel in such a fashion to protect and enclose the reflective arrays and transducers. Such contact may be effected by means of a resilient and water-tight seal, such as an RTV seal, between the touch panel substrate and the adjacent object. Such seals absorb acoustic wave energy, making it highly desirable to increase the signal-to-noise ratio prior to application of the seal.
(4) For many applications, assuring proper mechanical fit of a touch panel into a touch/display system involves optimizing the mechanical design at the expense of acoustic signal amplitudes. Mechanically compact transducers may be designed with less than optimal acoustic performance. Reflective arrays may be designed to be narrower than optimal for signal performance to accommodate mechanical constraints. The less signal that is lost due to damping in the substrate material, the more flexibility the design engineer has to improve mechanical fit at the expense of signal amplitude.
Due to its durability, scratch resistance and optical clarity, soda-lime glass has been the material of choice for acoustic touch panels. As noted above, borosilicate and barium-containing glasses provide these mechanical and optical advantages and at the same time, increase the signal-to-noise ratio. The experiments referred to above show a pronounced improvement over soda-lime glass.
The experiments were performed using the most important acoustic mode for acoustic touch panels: Rayleigh waves. As with soda-lime glass, other acoustic modes such as Lamb waves and shear waves may be caused to propagate in borosilicate or barium-containing glass substrates. A pattern has been observed of reduced acoustic attenuation in glasses with compositions that minimize the number of unconstrained broken links Sixe2x80x94Oxe2x88x92/Oxe2x88x92Si. There is every reason to believe that this general pattern is independent of acoustic mode.
The energy in Rayleigh waves is in the form of both shear and longitudinal stresses, strains and motions, and hence Rayleigh waves are subject to the damping mechanisms corresponding to these forms of energy. Shear waves have energy only in the form of shear strains, stresses and motion, and hence the damping mechanisms for shear waves are a subset of the damping mechanisms for Rayleigh waves. With the exception of a flexural wave, which contains only shear energy, Lamb waves have energy in both shear and longitudinal forms and hence share the same damping mechanisms, though in different proportions, as Rayleigh waves. Due to shared damping mechanisms, borosilicate and barium-containing glasses have reduced attenuation relative to soda-lime glass for all acoustic modes.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention.